Recently, mobile communication terminals such as portable telephones have widely spread. Such a mobile communication terminal is often utilized outdoor and hence it frequently suffers from surrounding noise such as traffic noise (noise generated around a person utilizing the mobile communication terminal). Therefore, if such noise becomes so serious, then it becomes often difficult for the user to hear the received speech. The reason why such phenomenon is brought about is, the surrounding noise effects a masking effect, which makes it difficult for the user to hear the received speech when the speech volume becomes lowered, with the result that the articulation is lowered.
As for the surrounding noise mixed into the speech signal transmitted from the transmission side, a mobile communication terminal on the transmission side is equipped with a noise canceller. Therefore, improvement can be expected in the countermeasure against the noise generated around the terminal of the transmission side.
However, it is impossible for the communication system to control the noise generated around the terminal of the receiver side. Therefore, the user of the mobile communication terminal who is talking in a surrounding noise environment will suffer from a hard situation in hearing the opponent's speech.
For this reason, as a countermeasure against the noise generated around the terminal of the receiver side, the user on the receiving side adjust the volume at which the user hears the received speech. Alternatively, the mobile communication terminal may be arranged to detect the surrounding noise level automatically so as to adjust the volume of the received speech in response to the detected surrounding noise level.
FIG. 20 is a block diagram of a speech signal transmitting receiving apparatus of a portable telephone apparatus disclosed in Japanese Patent Laid-open gazette No. HEI 9-130453. In an apparatus 42 shown in FIG. 20, a speech picked up by a transmitting microphone 36 converts the speech into an electric signal to create an input signal, and this input signal is formed into digital data by an analog-to-digital (A/D) converter 35. The digital data is supplied to a speech signal encoding unit 33 and transmitted from an antenna 30 through a baseband signal processing unit 32 and an RF transmitting receiving unit 31 as a radio signal.
Also, the output of the analog-to-digital converter 35 is supplied to a surrounding noise level detecting circuit 34. The surrounding noise detecting circuit 34 detects a surrounding noise level contained in a signal outputted from the analog-to-digital converter 35. Further, a received speech signal controlling circuit 37 adjusts a speed of increase and decrease of received speech volume in response to the increase and decrease of the surrounding noise level detected by the surrounding noise detecting circuit 34.
On the other hand, a radio signal received by the antenna 30 is supplied to a speech signal decoder 38 through the RF transmitting receiving unit 31 and the baseband signal processing unit 32. The speech signal having undergone decoding processing in the speech signal decoder 38 is converted into analog data by a digital-to-analog converter 39 and the resultant data is supplied to a variable gain amplifier 40. The variable gain amplifier 40 amplifies the volume of analog data in accordance with the speed of increase and decrease of the received speech volume adjusted by the received speech signal controlling circuit 37. Thus, a speech as a sound emanates from a speaker 42.
As described above, the speed of increase and decrease of the received speech volume is adjusted. Therefore, even if the surrounding noise level abruptly changes from a high level to a low level, the user can be protected from being brought into a state in which an ear of the user is temporarily made insensible in hearing the speech due to the surrounding noise level which has been kept so far. That is, the hearing environment of the user on the receiving side can be protected from being deteriorated. In this case, it is necessary for the communication terminal to be equipped with a circuit which discriminates a speech signal from the surrounding noise upon detecting the surrounding noise level, a circuit for detecting the surrounding noise level, or a circuit for adjusting the speed of increase and decrease of the received speech volume in response to the surrounding noise level, with the result that the size of the circuit could be enlarged.
Meanwhile, Japanese Patent Laid-open gazette No. HEI 8-163227 discloses a technology. That is, the surrounding noise level is detected when a talker on the transmitting side keeps quiet, whereby the level of the pure surrounding noise without the speech of the talker is detected and the received speech volume can be adjusted properly based on the detected noise level. In this case, of the signals picked up by the microphone, a signal falling in a speech band and a predetermined threshold value are compared with each other to determine whether the talker on the transmitting side is talking or listening to the opponent's talk. Therefore, it is necessary for the communication terminal to be provided with a circuit for recognition, with the result that the size of the circuit could be also enlarged.
Further, according to the above-described method in which the volume is adjusted in response to the surrounding noise level, in general, it is a complicated task to design a circuit concerning the detection or amplification of the surrounding noise level. Also, software development for the circuit is a hard task. Thus, the size of the circuit or software will be enlarged.
Furthermore, according to the above-described method, the volume of speech is uniformly amplified over the whole frequency bands, and hence the amplification can become excessive in a part of the frequency band. As a result, the speech can be distorted or the speech cannot provide a satisfactory articulation in other part of the frequency bands.
The present invention is made in view of the above aspect. Therefore, it is an object of the present invention to provide a received speech signal processing apparatus or a received speech signal reproducing apparatus in which the volume adjustment is effected in response not to the change of the surrounding noise level but to a frequency characteristic of the speech signal so that a fine adjustment concerning a speech adjustment or the like is obviated and further a complicated circuit such as a circuit for discriminating a speech signal from a non-speech signal or a circuit for controlling the processing of transmission and reception is also obviated, whereby the size of the circuit or the software can be made simple and a stable articulation can be obtained.